1. Field of the Invention
The present invention relates to a transmission screen to be used for projection televisions, microfilm readers, etc., a light diffusion plate to be used mainly therefor and its production process.
2. Discussion of Background
A projection television (PTV), particularly a rear-projection (rear type) PTV10 (see FIG. 4) of a system wherein projected light from an optical engine (projector) 11 is projected to the rear side of a screen 13 via a first-surface mirror 12 to transmit an enlarged image has attracted attention, which may realize a large screen for home theater and the like at a low cost. The above screen (transmission type) 13 comprises, as shown in FIG. 5, at least two lens sheets of a Fresnel sheet 2 which emits diffused light from the first-surface mirror as approximately parallel rays and a lenticular sheet 3 which emits the approximately parallel rays from the Fresnel sheet 2 as enlarged in the horizontal direction. Further, a protector 4 having functions of lens protection, low reflection/anti-glare, fingerprint removal, etc. is commonly disposed on the emission side of the lenticular sheet 3. The numerical symbol 31 designates a light shielding layer.
In the above screen for PTV, the horizontal viewing angle is enlarged by the lens action of the lenticular sheet 3, whereas enlargement of the vertical viewing angle utilizes light diffusion property of a light diffusion agent. As the light diffusion agent, a material containing fine particles having a particle size of about from 1 to 30 μm has been used. The embodiment of the material containing such fine particles, the fine particles may be kneaded in the protector 4, or a diffusion membrane containing the fine particles is laminated on the protector 4.
Further, along with the wide spread use of a microdisplay (MD) type high precision projector employing a matrix pixel structure such as liquid crystal or DMD (Digital Micromirror Device, tradename) as an optical engine in a PTV, unnecessary glare (flicker) phenomenon visible in a projected image, called scintillation, becomes outstanding. However, the principle of the scintillation, to what extent the scintillation is tolerable, and its quantitative evaluation method have not yet been established.
Light diffusion agents which can be used for an MD type PTV which involves problems of the scintillation phenomenon have been proposed. Particularly, it has been proposed to combine two or more different types of light diffusion agents so as to secure the original high precision image without decreasing the resolution.
For example, in a case where a light diffusion agent is incorporated in a substrate of the Fresnel sheet and/or the lenticular sheet, and each lens sheet has two light diffusion layers, it has been proposed that from the light transmission side, a light diffusion layer comprising an organic material having a uniform shape and then a light diffusion layer comprising an inorganic material such as silica are disposed in this order (Patent Document 1). Patent Document 1 discloses that by the above disposition, the light diffusion layer of the lenticular sheet closest to the observer is a light diffusion layer (hard coat) comprising an inorganic material which is in many cases in the form of non-uniform scales, whereby the surface of the light diffusion layer is properly roughened, and no glare is likely to occur. In Example, a hard coat layer (thickness: 20 μm) is formed by a ultraviolet-curable acrylic resin having silica dispersed therein at a concentration of 30% on one side of a transparent resin substrate by coating, and on the other side of the substrate, an organic layer (thickness: 20 μm) is formed from an acrylic resin having MS (acrylic/styrene copolymer) crosslinked beads dispersed at a concentration of 10% by coating, and the substrate is laminated on a lenticular sheet employing the organic layer as an adhesive layer.
As another example of combining an inorganic material and an organic material, disposition opposite to the above relative to the light transmission direction has been proposed (Patent Document 2). Patent Document 2 discloses an object to prevent hot spot phenomenon such as a local increase in the brightness of incident light from a projector and scintillation phenomenon (unnecessary flicker visible in the projected image) caused by a small exit pupil diameter of a projector lens of a PTV employing a high precision liquid crystal panel as compared with a CRT type PTV. It discloses, to is achieve the above object, a transmission screen comprising a first light diffusion sheet having a thickness of from 500 to 1,000 μm containing organic fine particles having an average particle size of from 5 to 15 μm in an amount of from 5 to 20 wt % in a base material, and a second light diffusion sheet having a thickness of from 50 to 500 (or 5,000) μm containing inorganic fine particles having an average particle size of from 2 to 10 μm in an amount of from 1 to 10 wt % in a base material. It discloses disposition of the first diffusion sheet on the observer side and the second diffusion sheet on the projector side (FIG. 2B of Patent Document 2).
Further, to provide a transmission screen which overcomes the problem of the hot spot phenomenon and the scintillation phenomenon in the same manner as in Patent Document 2, and which provides a bright image with high resolution, with large viewing angles in both horizontal and vertical directions, excellent in contrast and being clear, at a relatively low cost, a transmission screen has been proposed (Patent Document 3) in which at least one of the Fresnel sheet and the lenticular sheet has a structure of at least two layers differing in the concentration of dispersion of the light diffusion agent in the sheet substrate thickness direction. Patent Document 3 discloses that the light diffusion layer having a multilayer structure is suitable to desirably control the light diffusion property as the entire light diffusion substrate or depending upon the lens properties of the lens sheet, and specifically, it discloses a structure having two types of light diffusion layers having a thickness of 20 μm on a transparent substrate and a combination of inorganic materials or an inorganic material with an organic material as materials of the diffusion agent. However, it only discloses, as specific concentration distribution, a combination of a layer containing the diffusion agent in an amount of 30% and a layer containing it in an amount of 15%.
As another example of a transmission screen for the same purpose as the above transmission screen having a light diffusion layer in a multilayer structure differing in the concentration distribution of the light diffusion agent, a transmission screen having two layers of a first light diffusion layer (thickness: 50 to 200 μm) containing a light diffusion agent in an amount of from 20 to 50 wt % and a second light diffusion layer (thickness: 500 to 5,000 μm) containing a light diffusion agent in an amount of from 0.1 to 10.0 wt % has been disclosed (Patent Document 4). The light diffusion agent disclosed in Patent Document 4 is translucent fine particles having a weight average particle size of from 1 to 12 μm for each layer, and the translucent fine particles contained in the first light diffusion layer are limited to ones having a refractive index difference Δn of from 0.07 to 0.17 with the base material (translucent plastic), and the second light diffusion layer is defined to have a haze of from 50 to 85% of the entire layer. With respect to the above Δn of the translucent fine particles contained in the second light diffusion layer, a value of from 0.01 to 0.1 which is smaller than that in the first light diffusion layer is exemplified. The specific layer structure disclosed in Patent Document 4 is such a structure that on a thick substrate (second light diffusion layer) containing a light diffusion agent having a small Δn in a small amount (from 0.1 to 10 wt %), a light diffusion layer (first light diffusion layer) containing a light diffusion agent having a large Δn in a large amount (from 20 to 50 wt %) is laminated.
Use of a glass plate as a substrate on which a light diffusion layer is laminated has been proposed (Patent Document 5).
Patent Document 1: JP-A-2003-131325
Patent Document 2: JP-A-2003-131326
Patent Document 3: JP-A-2002-236319
Patent Document 4: JP-A-2000-180973
Patent Document 5: JP-A-2002-357868